The present invention generally relates to proton conducting polymer membranes and methods of manufacture thereof, which are useful in methanol-based fuel cells.
Proton conducting polymer membranes, or polymer electrolyte membranes, are of general interest because such membranes can be used to conduct protons in fuel cells, which convert methanol into electrical energy and show promise as low emission power sources. Methanol-based fuel cells produce power through the electrochemical reaction of methanol and oxygen whereby oxygen is reduced at the cathode and methanol is oxidized at the anode. An appropriate polymer membrane is insoluble in water and methanol and is selectively permeable to hydrogen ions.
Fluorocarbon based resins, such as NAFION(trademark) and its derivatives, are the most common materials used in the manufacture of solid-polymer electrolyte membranes in methanol fuel cells. The membranes are stable and conduct protons. However, the membranes are permeable to methanol and allow significant amounts of methanol to diffuse through the membrane and crossover from the anode to cathode resulting in the spontaneous oxidation of methanol at the cathode. This oxidation depletes fuel from the cell and results in a loss of energy and efficiency. In addition, NAFION(trademark) membranes are not cost efficient because NAFION(trademark) is an expensive starting material and the fabricated membranes become unusable upon dehydration at elevated temperatures.
It would therefore be advantageous to develop alternative membrane materials that are more resistant to methanol diffusion (and thus crossover) and which preferably are less expensive than fluorocarbon-based polymers. It would also be desirable to easily and efficiently substitute the polymer to alter, based on specific application needs, the bulk material properties of the polymer material forming the membrane.
It is therefore an object of the present invention to provide compositions for use in proton conducting membranes having enhanced methanol diffusion resistance, and methods of manufacture thereof.
It is another object of the present invention to provide proton conducting membranes having enhanced methanol diffusion resistance and improved bulk properties, preferably at a lower cost than fluorocarbon-based polymers.
Proton conducting membranes having improved resistance to methanol crossover are provided, along with methods for their manufacture. In a preferred embodiment, the polymeric membranes are formed by (a) dissolving a polymer, preferably a polyphosphazene, in an organic solvent to form a polymer solution; (b) adding an oxyacid to the polymer solution; (c) optionally, adding water to the polymer solution, preferably in a molar ratio equivalent to the oxyacid; (d) optionally, concentrating the polymer solution; (e) casting the polymer solution on a casting surface, such as one formed of or coated with TEFLON(trademark); and (f) removing the organic solvent, such as by a controlled evaporation, so as to form the polymeric membrane.
A particularly useful application for these polymeric membranes is in fuel cells, such as those wherein methanol and oxygen are converted into electrical energy.